The U.S. Pat. No. 5,085,161, of Cuneo et al., issued Feb. 4, 1992, discloses and claims a method and apparatus for fabricating hull modules for the longitudinal midbody of a double-walled vessel, and for serially interconnecting those modules to provide a midbody, to which prefabricated bow and stern modules are added to constitute the vessel hull. Improvements in the method and resulting vessel hull constructions are disclosed in the U.S. Pat. No. 5,090,351, of Goldbach et al, issued Feb. 25, 1992.
In the methods and constructions as disclosed in these prior patents, the fundamental fabricated subcomponent is two inner hull longitudinal plates arranged edge-to-edge so as to have a first set of two adjoining longitudinal edges, two outer hull longitudinal plates arranged edge-to-edge so as to have a second set of two adjoining longitudinal edges, and a longitudinal rib plate arranged generally perpendicularly to the inner and outer hull plates, with one longitudinal edge thereof juxtaposed with and forming part of the first set of adjoining edges, and the opposite longitudinal edge thereof juxtaposed with and forming part of the second set of adjoining edges. The first set of three plate edges is welded together to form a first T-joint and the second set of three plate edges is welded together to form a second T-joint. The resulting fabricated subcomponent is H-shaped and made up of five plates and two welds. In practice, according to the methods disclosed as preferred in the two prior patents, a subassembly which is more complex than the fundamental subcomponent is fabricated in up-ended orientation in a fixture in which more than five plates are simultaneously welded at more than two joints so as to simultaneously create and serially interconnect a plurality of such subcomponents. In these prior art constructions, the plates forming the inner and outer hull walls are disclosed as being convexly curved, and the hull wall interconnecting rib plates are disclosed as being flat, although possibly being provided with lightening hole and welded-on kick-plate stiffeners.
An earlier U.S. Pat. No. 4,638,754, to Tornay, issued Jan. 27, 1987 discloses a double-walled vessel hull construction, also having curved hull plates and flat rib plates, but in which the longitudinal edges of the rib plates are welded to the hull plates at locations intermediate the longitudinal edges of the curved inner and outer hull plates. Thus, in Tornay, each H-shaped subcomponent is made up of three (rather than five) plates, and these subcomponents are serially interconnected by welding two adjoining inner hull plate edges to one another and two outer hull plate edges to one another. The vessel hull construction of the Tornay patent is disclosed as needing transverse deep webs for avoiding excessive build up of transverse forces at the lower corner bilges of the vessel hull construction. Also, according to the Tornay patent, the curved hull plates should be recurved (so that they have a "sea gull" shape as seen in end view) rather than a simple edge-to-edge convex shape as disclosed in the above-mentioned, later Cuneo et al. and Goldbach et al. patents.
Among the advantages to be gained by using curved hull plates, is thereby obtaining some stiffening so as to reduce or eliminate the need for transverse ribs between the hulls. The curvature may also help accommodate application of jacking forces onto the plates in the welding fixture, so as to force joint edges into proper alignment and spacing for welding as the module subassemblies are fabricated.
Among the advantages to be gained by welding three plate edges together at a T-joint (rather than only two edges together at a butt joint, or a plate edge to a plate face at a two-plate T-joint as in the Tornay patent) are increased ability to use more highly automated welding processes, in which part of the necessary backing for the joint as it is being welded, is the edge of the third plate being united by the weld, the lower number of welds needed for producing a subassembly, and facilitation of applying jacking forces on the plates in the welding fixture for aligning and uniformly spacing their edges for welding.
The type of construction disclosed in the above-mentioned Cuneo et al. and Goldbach et al. patents was conceived mainly for use in very large crude oil carriers and similar bulk liquid cargo vessels which are normally filled and emptied of cargo by pumping. However, as disclosed, their use is not limited to that field.
As environmental and ecological considerations have risen and become more prominent in the design of tankers, causing more vessel owners and others concerned with shipping to seriously look towards greater adoption and use of double-hulled vessels for shipping bulk cargo, it has become clear that there are instances where curved vessel hull plates are at a disadvantage for use in certain hulls, or in portions of certain hulls.
A prime example is a bulk carrier for granular material such as rock salt or pulverized phosphate rock, or lumber. Holds are mainly emptied using clamshell buckets or slings on cranes, with the assistance of a wheeled front-end loader or forklift truck lowered into the hold for emptying corners and moving the material towards a location where it can be efficiently crane-lifted out of the hold. If the vessel midbody is made of curved plate as disclosed in the aforementioned Tornay, Cuneo et al. or Goldbach et al. patents, it will be difficult to drive a wheeled front-end loader or forklift truck around on the floor surface of the bottom of the hold, and particularly difficult to scoop up or plow granular material, due to the undulating pattern of ridges and troughs.